Application of dynamic braking to mine hoisting systems

Accidents occurred on counterweighted mine hoisting systems when the mechanical brake failed while the cage was empty. This allowed the counterweight to fall to the bottom of the shaft, causing the cage to overspeed and crash into the headframe. Direct-current hoist motors have the potential for preventing this type of accident by incorporating a passive electrical braking system known as dynamic braking. Installation of dynamic braking requires minimal control system modifications and modest expense. The various circuit designs that can be configured to implement dynamic braking on a DC mine hoisting system are discussed. Selection of the load resistance for the desired dynamic braking system performance is addressed. Data from tests of a mine hoisting system that was modified in the field to incorporate dynamic braking are analyzed, to provide technical credibility to the dynamic braking concept     

Safe electrical design of mine elevator control systems

A mine elevator recently experienced an ascending-car overspeed accident, resulting in serious injuries to four passengers. Although the four miners laid down on the floor prior to impact, the miners struck the ceiling of the elevator car as it collided into the overhead structure at an estimated speed four times faster than normal. Several electrical design precautions can be implemented to prevent elevator control system failures. This paper examines safe electrical design of elevator control systems. Supplemental circuits and devices which improve the safety integrity and maintenance of the elevator control system are presented. These circuits and devices provide protection that eliminates the potential hazard and significantly reduces the possibility of a mine elevator accident